The present invention relates to a material for a sliding contact to be used in an electrical, mechanically sliding element. More particularly, the invention relates to a material for a sliding contact to be used for a commutator of a small-sized direct current motor which, in turn, is used in a household electrical appliance driven on a rechargeable battery (or an earth ring, a rotary switch, or a like element).
Recently, in the above technical field, research efforts have been intensively carried out in an attempt to develop new materials for a sliding contact. A most significant subject in relation to development of a material for a sliding contact (hereinafter referred to as a sliding-contact material) is to realize ideal wear and low contact resistance when the material is applied to a contact. Low contact resistance of a sliding-contact material is realized by not only high electrical conductivity of the material but also secure or close contact between materials that contact. However, in the case where the contacting materials slide on each other, frictional resistance increases with the degree of contact between the contacting materials. Sliding against such friction causes a conspicuous wear phenomenon. Thus, there cannot be obtained a sliding-contact material that exhibits ideal characteristics unless the above contradictory phenomena are controlled. The wear phenomenon of a sliding contact involves many factors which are not yet scientifically solved; thus, control of the wear phenomenon is said to be very difficult to achieve through the improvement of a sliding-contact material.
Wear on a sliding-contact material is roughly classified into adhesive wear and scratch wear. Generally, even when the surface of a sliding-contact material is finished to a considerably high degree of smoothness, the surface is not a perfect plane on a microscopic scale, but has many fine pits and projections. Such metallic surfaces, when brought into contact with each other, appear to contact over a wide area. However, in actuality, only fine projections present on the surfaces are in contact with each other. Thus, a so-called true contact area is smaller than an apparent contact area. Accordingly, a large pressure is imposed on the true contact portion, i.e., on contacting projections, causing fusion between the contacting metals. As a result, a softer metal is torn of f and transfers onto the other harder metal; that is, adhesive wear occurs. When materials having different hardnesses are in contact with each other or with soft metals, either of which makes hard contact with each other, hard metal mechanically shears soft metal, causing scratch wear.
Such a wear phenomenon depends on the hardness of contacting metallic materials and the state of engagement of the metallic materials. Basically, a wear phenomenon of a sliding-contact material grows in proportion to contact pressure and becomes less intensive with hardening of material. However, the wear phenomenon changes greatly depending on a temperature during contact, variation in humidity, and the presence of a corrosive component, organic vapor, dust, or a like substance. Such change in the wear phenomenon is variation in the state of contact at a contact portion and thus induces an increase in contact resistance, thus significantly affecting stable maintenance of low contact resistance.
In the case of a small-sized direct current motor whose commutator is of a composite clad material using a sliding-contact material, the above-mentioned wear phenomenon occurs between the commutator and a brush when the motor runs at high speed. Specifically, being subjected to contact friction over a long period of time and frictional heat induced by sliding, a sliding-contact material, which constitutes the commutator, suffers adhesive wear and scratch wear simultaneously. As a result, the surface of the sliding-contact material is grated, thereby generating wear powder. The thus-generated wear powder causes an increase in contact resistance, becomes caught in gaps of the commutator to thereby cause a short circuit, or cause noise.
In the case of a composite clad material using a sliding-contact material, the progress of such a wear phenomenon causes destruction of metal provided in a surface layer of the composite clad material, i.e., destruction of the sliding-contact material. As a result, wear reaches a base material located under the sliding-contact material; that is, the base material, which oxidizes easily, is exposed. A metallic oxide generated through oxidation of the base material may cause various electrical problems. Therefore, when a two-layer or three-layer composite clad material is to be used as a commutator, the improvement of an alloy material constituting each layer is a very important task to undertake.
Examples of materials recently used for a commutator for a small-sized direct current motor used in a household electrical appliance driven on a rechargeable battery, i.e., examples of a sliding-contact material, include a two-layer composite clad material (for example, Ag99--Cd1/Cu) that uses as a surface layer a Ag--Cd alloy containing 1-2% by weight Cd and the balance Ag and uses as a base layer Cu or a Cu alloy; and a two-layer composite clad material (for example, Ag97.7-Cd2-Ni0.3/Cu) that uses as a surface layer an Ag--Cd--Ni alloy containing 1-2% by weight Cd, 0.01-0.7% by weight Ni, and the balance Ag and uses as a base layer Cu or a Cu alloy. The above parenthesized expression "[alloy composition]/Cu" denotes a two-layer composite clad material, and the symbol "/" denotes the boundary between a surface layer and a base layer. In the expression "[alloy composition]/Cu," a number appearing after a component element of the alloy composition is a value in the unit of % by weight.
Such Ag--Cd and Ag--Cd--Ni alloys exhibit excellent electrical functions, hardness, and contact resistance. For example, Japanese Patent Publication No. 2-60745 discloses a Ag alloy serving as a sliding-contact material for a commutator of a small-sized direct current motor. The Ag alloy contains Sn and/or Cd in a total amount of 1-5% by weight and the balance Ag. However, since Cd is a detrimental substance in terms of effects on environment, manufacture or use of a sliding-contact material containing Cd is not preferred.
Other alloys serving as sliding-contact materials include Ag--Cu and Ag--Cu--Cd. However, these materials have a problem in that contact resistance is low at an early stage of use but exhibits time-course variation, resulting in deterioration in product value for electric razors and like products using a rechargeable battery. Specifically, when a motor employs such an alloy as a sliding-contact material, an increase in contact resistance results from time-course variation, causing an increase in motor starting voltage. Accordingly, the electromotive force of a battery decreases; consequently, the motor does not start up. As a result, the frequency of battery recharge increases, and the battery life itself tends to shorten.
Also, for example, Japanese Patent Application Laid-Open (kokai) No. 58-104140 discloses an Ag--Zn alloy serving as a sliding-contact material. The Ag--Zn alloy contains Zn in an amount of 1-10% by weight, at least one element selected from a group consisting of Te, Co, Ni, Cu, Ge, Ti, and Pb in a total amount of 0.5-1.0% by weight, and the balance Ag. Since Te, Co, Ni, Cu, Ge, Ti, and Pb oxidize more easily than does Zn, such an element(s) is contained in the sliding-contact material in an attempt to suppress oxidation of Zn so that the sliding-contact material maintains sulfurization resistance and lubricity and exhibits improved wear resistance and stable, low contact resistance. However, as in the case of the above Ag--Cu alloy, the contact resistance of this sliding-contact material is low at an early stage of use, but exhibits time-course variation.
Further, Japanese Patent Application Laid-Open (kokai) No. 8-260078 discloses Ag--Zn and Ag--Zn--Ni alloys serving as sliding-contact materials. Again, these sliding-contact materials exhibit low contact resistance, but cannot control a wear phenomenon to such a sufficient extent as to improve the life of a motor.
As described above, the conventional sliding-contact materials are not sufficiently compatible with recent household electrical appliances which are driven on a rechargeable battery. Recent household electrical appliances using a rechargeable battery, particularly headphone stereos, cameras, and electric razors, are required to enable continuous use over a long period of time even when a motor having low starting voltage and a rechargeable battery are used. Thus, there has been great demand for a sliding-contact material that enables realization of such products.
In view of the foregoing, an object of the present invention is to provide a sliding-contact material whose alloy composition does not contain an environmentally detrimental substance such as Cd, which exhibits excellent contact resistance, whose electrical functions are good and free of time-course variation, and whose wear resistance practically compares with that of conventional sliding-contact materials, as well as to extend the life of a small-sized direct current motor through use of the sliding-contact material having such excellent characteristics in a commutator thereof.